The long term objective of this proposal is the generation of a mouse model for cystic fibrosis (CF). Mutations known to cause CF in humans will be introduced into the murine CFTR gene of pluripotent embryonic stem (ES) cells. Mouse lines will be derived from ES cells carrying these mutations and evaluated for the presence of disease similar to that characteristic of CF in humans. The extent to which the pathophysiology of the disease in mice mimics that in humans will define whether the mouse model will be useful for testing therapeutic protocols, including gene therapy. We have six specific aims: 1. We will generate a mouse line carrying a CFTR gene in which a termination codon has been introduced into exon 10 by gene targeting. 2. We will generate a mouse line carrying the most common mutation found in human CF patients, the deletion of three bases in exon 10. 3. We will generate mice carrying the A117H mutation in exon 4, which is known to result in a mild form of CF in humans. 4. During the course of generating these three animals we will define factors which affect the targeting of DNA to the CFTR locus. 5. We will generate mice in which the expression of the CFTR gene can be detected with a chromogenic assay to assist in the analysis of disease in these animals. 6. We will evaluate the disease caused by the three CFTR mutations to determine whether the initiation and progression of the disease are similar to those in humans. This will determine to what extent these animals can be used for in vivo analysis of the pathophysiology of CF and for testing new approaches to the treatment of CF.